The randomized controlled trial (RCT) is the study design with the greatest potential to maximize internal validity when assessing the effectiveness of medical interventions, making it invaluable for evidence-based medicine. Yet, especially in the field of rehabilitation, it is not universally accepted as an unassailable gold standard due to serious problems of its implementation. This paper first examines three factors that limit the applicability of RCTs in rehabilitation practice. The first two factors stem from the nature of rehabilitative treatment itself: the complexity of rehabilitation interventions and the long-term and holistic nature of rehabilitation goals. The third factor relates to the differing functions of RCTs. Interventions vary in their complexity in increasing degree between component, measure, and program interventions. Lower complexity is associated with a greater likelihood of using high rigor efficacy studies. Methodological rigor further depends on the degree to which intervention conditions or contexts can be controlled for. This is particularly the case when examining body-related short-term outcomes. Whether it is reasonable to conduct an RCT also hinges on its function: to gain knowledge or to legitimate the utilization of an intervention in rehabilitation practice. The discussion highlights key challenges to RCT implementation and states questions that should help to identify an RCT as the most appropriate research design. Further empirical and theoretical research is indicated to clarify the distinction between levels of intervention, as this paper is based on theoretical considerations. Additionally, a concise explication of the different functions of an RCT and its meanings for their implementation is needed.